In 3GPP LTE Release 8, Release 9 and Release 10 specifications, each downlink subframe includes a control area and a data area with 14 OFDM (Orthogonal Frequency Division Multiplexing) symbols. The first one to four OFDM symbols of each subframe may serve as the control area, which includes a PCFICH (Physical Control Format Indicator Channel), a PDCCH (Physical downlink control channel), a PHICH (Physical Hybrid HARQ indicator channel), CRSs (Common reference signal) and some blank REs (Resource Element). DCI (Downlink Control Information) carried in the PDCCH includes resource allocation information and other control information sent to one or more UEs (User Equipment). Generally, one subframe may include several PDCCHs, each corresponding to a different UE.
Each PDCCH may be dispersed over the entire bandwidth and all the OFDM symbols within the control area. In accordance with user's channel conditions, the PDCCH may occupy 1, 2, 4 or 8 CCEs (Control Channel Element) adaptively, and each CCE includes 36 REs. For example, for the user with a well channel condition, the PDCCH may occupy 1 or 2 CCEs, and for the user with a bad channel condition, the PDCCH may occupy 4 or 8 CCEs. To be specific, a base station calculates a SINR (Signal to Interference plus Noise Ratio) in accordance with a wideband CQI (Channel Quality Indicator) reported by the UE, and then calculates the number of CCEs occupied by the PDCCH of the UE in accordance with the SINR.
However, the inventor finds that there exists at least the following defect in the prior art during the implementation of the present invention.
The transmission mode for the control area (e.g., 2×2 SFBC, Space Frequency Block Code) is different from that for the data area (including 9 transmission modes, i.e., Transmission mode 1 to Transmission mode 9), and the inter-cell interference over the control area is also different from that over the data area, so the wideband CQI reported by the UE can merely reflect the channel quality of the data area rather than the control area. In addition, there exists a need of reliable PDCCH transmission, e.g., an error rate of the PDCCH transmission shall usually be less than 1%. It is an unstable and rough method to perform the PDCCH link adaptation merely in accordance with the wideband CQI reported by the UE (i.e., selecting 1, 2, 4 or 8 CCEs to transmit the PDCCH). By using such a method, it is very difficult to ensure that the error rate of the PDCCH transmission is less than 1%.